A novel method of allelic exchange has been developed. This method is demonstrated in E. coli with the use of a mutant luciferase gene, or mutant allele. The efficiency of this novel method in exchanging the mutant allele for the normal luciferase allele is about two percent. This technique is portable and will be tried in mammalian cell lines in the near future. We hope to improve the efficiencies beyond two percent. Once accomplished, it becomes reasonable to replace a mutant allele with a normal allele, without selecting one population of cells from the other, both in vitro and in vivo in mammals.